Hydrogen Sulfide smell coming from sink and shower drain??

One of my renters says he smells hydrogen sulfide (his diagnosis) coming from his bathroom sink and shower drains "occasionally". He is familiar with the smell as he tests water as part of his job duties so I guess he knows what he is talking about. Assuming he is correct, what would cause this odor and what can I do to to get rid of it?? Any suggestions would be greatly appreciated.

At least two possiblities;
1. the odor is coming from the hot water
2. The sink's overflow pipe is "fouled" and turning the faucet on forces the odor into the room.
3. Unless the traps have failed it is NOT coming through the drain lines.

I'm going to guess that it is coming from the drain, not from the faucet. There is probably a bacteria culture established in the soap scum residue. Pouring bleach probably would knock it back for a while but it is likely to return unless the soap scum is cleaned out.

I had the problem in my shower drain and was pouring some bleach in it every week. I finally got rid of it by running a brush down it. I used one of those 3 inch brushes made to get dust out of a dryer vent.

I have also had bacteria get established in the vinyl hose of the hand sprayer at the kitchen sink. I took it off and ran a wad of cloth with bleach through it like one would do cleaning a gun. My cold supply line to my bathroom vanity also had the same issue which was resolved the same way. I keep my hot water temp high enough that bacteria cannot establish on the hot supply. The bacteria never did come back, so I'm guessing I contaminated it during installation and that there is not a bacteria problem in my water.